I thought I’d post this information in case it helps someone else. I was having issues with Kdenlive where I would get a glibc double free crash at the end of the rendering process. It was consistent and very annoying as you can imagine. Looking on-line, I found similar problems with the OpenCV Frei0r plugins (which I didn’t have installed), similar problems with previous versions of melt (Kdenlive uses it for rendering), and so on.
Turns out my problem was with having clips with spaces in the file names. On my Fedora 19 system (x86-64, Kdenlive 0.9.6.2, mlt 0.8.8.5, ffmpeg 188.8.131.52) this appears to have been the root of the problem since removing the spaces makes things consistently render without errors. Might give this a try if you’re having problems out there.